A round robin on room acoustical simulation and auralization

Auditory orientation and distance estimation of sighted humans using virtual echolocation with artificial and self-generated sounds

Active echolocation of sighted humans using predefined synthetic and self-emitted sounds, as habitually used by blind individuals, was investigated. Using virtual acoustics, distance estimation and directional localization of a wall in different rooms were assessed. A virtual source was attached to either the head or hand with realistic or increased source directivity. A control condition was tested with a virtual sound source located at the wall. Untrained echolocation performance comparable to performance in the control condition was achieved on an individual level. On average, the echolocation performance was considerably lower than in the control condition, however, it benefitted from increased directivity.

Binaural detection thresholds and audio quality of speech and music signals in complex acoustic environments

Every-day acoustical environments are often complex, typically comprising one attended target sound in the presence of interfering sounds (e.g., disturbing conversations) and reverberation. Here we assessed binaural detection thresholds and (supra-threshold) binaural audio quality ratings of four distortions types: spectral ripples, non-linear saturation, intensity and spatial modifications applied to speech, guitar, and noise targets in such complex acoustic environments (CAEs). The target and (up to) two masker sounds were either co-located as if contained in a common audio stream, or were spatially separated as if originating from different sound sources. The amount of reverberation was systematically varied. Masker and reverberation had a significant effect on the distortion-detection thresholds of speech signals. Quality ratings were affected by reverberation, whereas the effect of maskers depended on the distortion. The results suggest that detection thresholds and quality ratings for distorted speech in anechoic conditions are also valid for rooms with mild reverberation, but not for moderate reverberation. Furthermore, for spectral ripples, a significant relationship between the listeners' individual detection thresholds and quality ratings was found. The current results provide baseline data for detection thresholds and audio quality ratings of different distortions of a target sound in CAEs, supporting the future development of binaural auditory models.

Acoustically driven orientation and navigation in enclosed spaces

Awareness of space, and subsequent orientation and navigation in rooms, is dominated by the visual system. However, humans are able to extract auditory information about their surroundings from early reflections and reverberation in enclosed spaces. To better understand orientation and navigation based on acoustic cues only, three virtual corridor layouts (I-, U-, and Z-shaped) were presented using real-time virtual acoustics in a three-dimensional 86-channel loudspeaker array. Participants were seated on a rotating chair in the center of the loudspeaker array and navigated using real rotation and virtual locomotion by "teleporting" in steps on a grid in the invisible environment. A head mounted display showed control elements and the environment in a visual reference condition. Acoustical information about the environment originated from a virtual sound source at the collision point of a virtual ray with the boundaries. In different control modes, the ray was cast either in view or hand direction or in a rotating, "radar"-like fashion in 90° steps to all sides. Time to complete, number of collisions, and movement patterns were evaluated. Navigation and orientation were possible based on the direct sound with little effect of room acoustics and control mode. Underlying acoustic cues were analyzed using an auditory model.

A filter representation of diffraction at infinite and finite wedges

Diffraction of sound occurs at sound barriers, building and room corners in urban and indoor environments. Here, a unified parametric filter representation of the singly diffracted field at arbitrary wedges is suggested, connecting existing asymptotic and exact solutions in the framework of geometrical acoustics. Depending on the underlying asymptotic (high-frequency) solution, a combination of up to four half-order lowpass filters represents the diffracted field. Compact transfer function and impulse response expressions are proposed, providing errors below ±0.1 dB. To approximate the exact solution, a further asymptotic lowpass filter valid at low frequencies is suggested and combined with the high-frequency filter.

Reduced basis methods for numerical room acoustic simulations with parametrized boundaries

The use of model-based numerical simulations of wave propagation in rooms for engineering applications requires that acoustic conditions for multiple parameters are evaluated iteratively, which is computationally expensive. We present a reduced basis method (RBM) to achieve a computational cost reduction relative to a traditional full-order model (FOM) for wave-based room acoustic simulations with parametrized boundaries. The FOM solver is based on the spectral-element method; however, other numerical methods could be applied. The RBM reduces the computational burden by solving the problem in a low-dimensional subspace for parametrized frequency-independent and frequency-dependent boundary conditions. The problem is formulated in the Laplace domain, which ensures the stability of the reduced-order model (ROM). We study the potential of the proposed RBM in terms of computational efficiency, accuracy, and storage requirements, and we show that the RBM leads to 100-fold speedups for a two-dimensional case and 1000-fold speedups for a three-dimensional case with an upper frequency of 2 and 1 kHz, respectively. While the FOM simulations needed to construct the ROM are expensive, we demonstrate that the ROM has the potential of being 3 orders of magnitude faster than the FOM when four different boundary conditions are simulated per room surface.

Feasibility of a finite-difference time-domain model in large-scale acoustic simulations

Wave-based techniques for room acoustics simulations are commonly applied to low frequency analysis and small-sized simplified environments. The constraints are generally the inherent computational cost and the challenging implementation of proper complex boundary conditions. Nevertheless, the application field of wave-based simulation methods has been extended in the latest research decades. With the aim of testing this potential, this work investigates the feasibility of a finite-difference time-domain (FDTD) code simulating large non-trivial geometries in wide frequency ranges. A representative sample of large coupled-volume opera houses allowed demonstration of the capability of the selected FDTD model to tackle such composite geometries up to 4 kHz. For such a demanding task, efficient calculation schemes and frequency-dependent boundary admittances are implemented in the simulation framework. The results of in situ acoustic measurements were used as benchmarks during the calibration process of three-dimensional virtual models. In parallel, acoustic simulations performed on the same halls through standard ray-tracing techniques enabled a systematic comparison between the two numerical approaches highlighting significant differences in terms of input data. The ability of the FDTD code to detect the typical acoustic scenarios occurring in coupled-volume halls is confirmed through multi-slope decay analysis and impulse responses' spectral content.

Perceptual Matching of Room Acoustics for Auditory Augmented Reality in Small Rooms - Literature Review and Theoretical Framework

For the realization of auditory augmented reality (AAR), it is important that the room acoustical properties of the virtual elements are perceived in agreement with the acoustics of the actual environment. This perceptual matching of room acoustics is the subject reviewed in this paper. Realizations of AAR that fulfill the listeners' expectations were achieved based on pre-characterization of the room acoustics, for example, by measuring acoustic impulse responses or creating detailed room models for acoustic simulations. For future applications, the goal is to realize an online adaptation in (close to) real-time. Perfect physical matching is hard to achieve with these practical constraints. For this reason, an understanding of the essential psychoacoustic cues is of interest and will help to explore options for simplifications. This paper reviews a broad selection of previous studies and derives a theoretical framework to examine possibilities for psychoacoustical optimization of room acoustical matching.

Binaural reproduction of dummy head and spherical microphone array data-A perceptual study on the minimum required spatial resolution

Dynamic binaural synthesis requires binaural room impulse responses (BRIRs) for each head orientation of the listener. Such BRIRs can either be measured with a dummy head or calculated from the spherical microphone array (SMA) data. Because the dense dummy head measurements require enormous effort, alternatively sparse measurements can be performed and then interpolated in the spherical harmonics domain. The real-world SMAs, on the other hand, have a limited number of microphones, resulting in spatial undersampling artifacts. For both of the methods, the spatial order N of the underlying sampling grid influences the reproduction quality. This paper presents two listening experiments to determine the minimum spatial order for the direct sound, early reflections, and reverberation of the dummy head or SMA measurements required to generate the horizontally head-tracked binaural synthesis perceptually indistinguishable from a high-resolution reference. The results indicate that for direct sound, N = 9-13 is required for the dummy head BRIRs, but significantly higher orders of N = 17-20 are required for the SMA BRIRs. Furthermore, significantly lower orders are required for the late parts with N = 4-5 for the early reflections and reverberation of the dummy head BRIRs but N = 12-13 for the early reflections and N = 6-9 for the reverberation of the SMA BRIRs.

Spatial Resolution of Late Reverberation in Virtual Acoustic Environments

Late reverberation involves the superposition of many sound reflections, approaching the properties of a diffuse sound field. Since the spatially resolved perception of individual late reflections is impossible, simplifications can potentially be made for modelling late reverberation in room acoustics simulations with reduced spatial resolution. Such simplifications are desired for interactive, real-time virtual acoustic environments with applications in hearing research and for the evaluation of hearing supportive devices. In this context, the number and spatial arrangement of loudspeakers used for playback additionally affect spatial resolution. The current study assessed the minimum number of spatially evenly distributed virtual late reverberation sources required to perceptually approximate spatially highly resolved isotropic and anisotropic late reverberation and to technically approximate a spherically isotropic sound field. The spatial resolution of the rendering was systematically reduced by using subsets of the loudspeakers of an 86-channel spherical loudspeaker array in an anechoic chamber, onto which virtual reverberation sources were mapped using vector base amplitude panning. It was tested whether listeners can distinguish lower spatial resolutions of reproduction of late reverberation from the highest achievable spatial resolution in different simulated rooms. The rendering of early reflections remained unchanged. The coherence of the sound field across a pair of microphones at ear and behind-the-ear hearing device distance was assessed to separate the effects of number of virtual sources and loudspeaker array geometry. Results show that between 12 and 24 reverberation sources are required for the rendering of late reverberation in virtual acoustic environments.

Fast multipole accelerated boundary element methods for room acoustics

Direct and indirect boundary element methods, accelerated via the fast multipole method, are applied to numerical simulation of room acoustics for rooms of volume ∼150 m³ and frequencies up to 5 kHz on a workstation. As the parameter kD (wavenumber times room diameter) is large, stabilization of the previously developed fast algorithms is required for accuracy. A stabilization scheme is one of the key contributions of this paper. The computations are validated using well-known image source solutions for shoebox shaped rooms. Computations for L-shaped rooms are performed to illustrate the ability to capture diffractions. The ability to model in-room baffles and boundary openings (doors/windows) is also demonstrated. The largest case has kD > 1100 with a discretization of size 6 × 10⁶ elements. The performance of different boundary integral formulations was compared, and their rates of convergence using a preconditioned flexible generalized minimal residual method were found to be substantially different. These promising results suggest a path to efficient simulations of room acoustics via high performance boundary element methods.

Discrimination of 2D wall textures by passive echolocation for different reflected-to-direct level difference configurations

In this work, we study people's ability to discriminate between different 2D textures of walls by passive listening to a pre-recorded tongue click in an auralized echolocation scenario. In addition, the impact of artificially enhancing the early reflection magnitude by 6dB and of removing the direct component while equalizing the loudness was investigated. Listening test results for different textures, ranging from a flat wall to a staircase, were assessed using a 2 Alternative-Forced-Choice (2AFC) method, in which 14 sighted, untrained participants were indicating 2 equally perceived stimuli out of 3 presented stimuli. The average performance of the listening subjects to discriminate between different textures was found to be significantly higher for walls at 5m distance, without overlap between the reflected and direct sound, compared to the same walls at 0.8m distance. Enhancing the reflections as well as removing the direct sound were found to be beneficial to differentiate textures. This finding highlights the importance of forward masking in the discrimination process. The overall texture discriminability was found to be larger for the walls reflecting with a higher spectral coloration.

The role of early and late reflections on perception of source orientation

Sound radiation of most natural sources, like human speakers or musical instruments, typically exhibits a spatial directivity pattern. This directivity contributes to the perception of sound sources in rooms, affecting the spatial energy distribution of early reflections and late diffuse reverberation. Thus, for convincing sound field reproduction and acoustics simulation, source directivity has to be considered. Whereas perceptual effects of directivity, such as source-orientation-dependent coloration, appear relevant for the direct sound and individual early reflections, it is unclear how spectral and spatial cues interact for later reflections. Better knowledge of the perceptual relevance of source orientation cues might help to simplify the acoustics simulation. Here, it is assessed as to what extent directivity of a human speaker should be simulated for early reflections and diffuse reverberation. The computationally efficient hybrid approach to simulate and auralize binaural room impulse responses [Wendt et al., J. Audio Eng. Soc. 62, 11 (2014)] was extended to simulate source directivity. Two psychoacoustic experiments assessed the listeners' ability to distinguish between different virtual source orientations when the frequency-dependent spatial directivity pattern of the source was approximated by a direction-independent average filter for different higher reflection orders. The results indicate that it is sufficient to simulate effects of source directivity in the first-order reflections.

Improved Room Acoustics Quality in Meeting Rooms: Investigation on the Optimal Configurations of Sound-Absorptive and Sound-Diffusive Panels

This work deals with the improvement of the room acoustic quality of two medium sized meeting rooms through the investigation of the optimal placement of absorption and diffusive panels on the walls and ceiling. Acoustic measurements have been carried out in the existing untreated rooms with ODEON 13 room acoustics measurement and prediction software, and the Adobe Audition plugins Aurora. Simulations of different combinations of sound absorption and diffusion treatments have been carried out with the updated version of the software, ODEON 15. The panels were positioned in the meeting rooms following the guidelines of the DIN 18041 standard and the scientific literature. The results advise the application of absorptive materials on the ceiling or around the borders, creating a reflective middle area, and on the upper part of one the lateral walls, including the rear wall. Configurations with diffusers do not generally bring significant improvements. The Speech Transmission Index (STI) is a less sensitive parameter for the different acoustic scenarios, compared to Reverberation Time (T) and Clarity (C50). The research also outlined a design workflow, useful to successfully design meeting rooms and rooms for speech in general, which allows to determine the optimal number and location of acoustic panels and to minimize the costs.

Introduction to the Special Issue on Room Acoustic Modeling and Auralization

This Special Issue on Room Acoustic Modeling and Auralization contains nineteen research papers. A majority of the papers focus on various room acoustic simulation techniques, while the remaining ones concentrate on auralization of either simulation or measurement results. Using room acoustic simulation, the last paper in this issue presents a case study of the historic venue, Palais du Trocadero in Paris, France.